The present invention concerns the performance of loopback tests upon data transmission paths operating asynchronously in fully duplex manner, and in particular concerns such data transmission paths wherein the maximun rate of data transmission in one direction differs greatly from the maximum rate of data transmission in the other direction.
In the case of data transmission paths operating asynchronously in fully duplex manner, it is frequently necessary to utilize a loopback test circuit to perform a test-signal transmission in order to evaluate the quality of data transmission along such data transmission path. FIG. 1 schematically depicts a set-up for the performance of such a loopback test. A predetermined rectangular binary test waveform is applied to the transmitting input 1 of station 2, and transmitted in suitable form (e.g., modulated onto a carrier) from station 2 along transmission channel 3 to a remote station 4. The test waveform, as it appears at the receiver output 5 of remote station 4, is looped back to the transmitting input 6 of station 4 and transmitted via transmission channel 7 back to proximate station 2. Evaluation of the quality of the data transmission duplex path is then performed by resort to factors such as error rate, waveform distortion with respect to time, and so forth, in conventional manner; this involves comparison between the waveform reconstituted at the receiving output 8 of proximate station 2 and the test waveform originally applied to the transmitting input 1 of station 2.
The performance of such a loopback test begins to be problematic when the two data transmission channels 3 and 7 are designed for markedly different respective data transmission rates. An example of such data transmission paths are those which operate with modems, i.e., are provided with a modulator and demodulator at each of the two stations at the two ends of the duplex path, with the main data transmission channel of the transmission path designed for a maximum data transmission rate of 1200 bits/sec, but with the opposite-direction transmission channel designed for use as an auxiliary channel having a maximum data transmission rate of only 75 bits/sec.
It is conceivable to perform a loopback test upon such a duplex transmission path utilizing for the test waveform a data transmission rate corresponding to the lower of the two data transmission rates. The disadvantage of such an approach, however, is that the main transmission channel is not tested under realistic conditions corresponding to normal use. For example, such a test may not bring to light problems such as high distortion which sets in only with the onset of high data transmission rates.
Accordingly, if both the main and auxiliary channel are each to be tested using their respective maximum data transmission rates, this could be achieved by transmitting a high-rate test waveform through the main channel 3 and then, at remote station 4, evaluating the received waveform and generating at station 4 a receipt or handshaking signal which indicates the result of the main-channel test and transmitting such signal back to proximate station 2 via auxiliary channel 7. However, such a technique would necessitate the provision at remote station 4 of circuitry for generating a receiver clock signal and synchronizing it, inasmuch as this would be necessary for evaluation of the arriving test waveform. Providing asynchronously operating data transmission devices with such synchronizing circuitry merely for test purposes would, however, lead to an unacceptable cost increase.